The present invention pertains to the art of diagnostic imaging. It finds particular application in conjunction with CT scanners for generating images of interior regions of human patients and will be described with particular reference thereto. However, it is to be appreciated, that the present invention will also find application in conjunction with industrial CT, quality assurance, and other types of x-ray diagnostic imaging, x-ray generation applications, and the like.
Typically, a patient is positioned in a prone position on a horizontal couch through a central bore of a CT scanner. An x-ray tube mounted on a rotatable gantry portion is rotated around the patient at a high rate of speed. For faster scans, the x-ray tube is rotated more quickly. However, rotating the x-ray tube more quickly decreases the net radiation per image unless the x-ray output of the x-ray tube is increased. As CT scanners have become faster, larger x-ray tubes which generate more radiation per unit time have been required. The high gantry rotational speeds, of course, cause high inertial forces during rotation.
High performance x-ray tubes for CT scanners and the like commonly include a stationary cathode and a rotating anode disk, both enclosed within an evacuated housing. When higher intensity x-ray beams are generated, there is more heating of the anode disk. In order to provide sufficient time for the anode disk to cool by radiating heat through the vacuum to surrounding fluids, x-ray tubes with progressively larger anode disks have been built.
The larger anode disks require larger x-ray tubes which do not readily fit in the small confined spaces of existing CT scanner gantries In a fourth generation scanner, the incorporation of a larger x-ray tube and heavier duty support structure requires moving the radiation detectors to a larger diameter. If the distance from the x-ray focal spot to the collimator is too short, the x-ray penumbra and beam divergence cause a degradation in image quality. Not only is a larger x-ray tube required, larger heat exchange structures are required to remove the larger amount of heat which is generated. Thus, as the CT scanners have become faster, they have become more massive, hence more difficult to move and install.
Rather than rotating a single x-ray tube around the subject, others have proposed using a switchable array of x-ray tubes, e.g. five or six x-ray tubes in a ring around the subject. However, unless the tubes rotate only limited data is generated and only limited image resolution is achieved. If the x-ray tubes rotate, similar mechanical problems are encountered trying to move all the tubes quickly.
Still others have proposed constructing an essentially bell-shaped, evacuated x-ray tube envelope with a mouth that is sufficiently large that the patient can be received in the well of the tube. An x-ray beam source is disposed at the apex of the bell to generate an electron beam which impinges on an anode ring at the mouth to the bell. Electronics are provided for scanning the x-ray beam around the evacuated bell-shaped envelope. One problem with this design is that it is only capable of scanning about 210.degree.. Another problem is that the very large evacuated space required for containing the scanning electron beam is difficult to maintain in an evacuated state. Troublesome and complex vacuum pumping systems are required. Another problem is that no provision can be made for off-focus radiation. Another problem resides in its large physical size.
Messrs. Mayden, Shepp, and Cho in "A New Design For High-Speed Computerized Tomography", IEEE Transactions on Nuclear Science, Vol. NS-26, No. 2, April 1979, proposed reducing the size of the conical or bell-shaped tube discussed above by rotating the cathode around the large diameter anode ring. However, their design had several engineering deficiencies and was not commercially produced.
The present invention contemplates a new and improved CT scanner which overcomes the above-referenced problems and others.